The aim of this proposal will be to establish a biochemically rational basis for the selective eradication of Ara-C-resistant human leukemic cells and to initiate a clinical trial in leukemia patients refractory to high dose Ara-C therapy based upon this strategy. This approach will exploit an enzymatic perturbation capable of conferring high degrees of Ara-C resistance, specifically loss of deoxycytidine kinase activity. The inability of the naturally-occuring nucleoside deoxycytidine to protect a deoxycytidine kinase-deficient, Ara-C-resistant human leukemic cell line (HL-60/ARA-C) from the lethal actions of ribonucleotide reductase inhibitors will be examined utilizing soft agar cloning techniques. Deoxycytidine concentrations and schedules will be sought which are capable of protecting normal human bone marrow progenitors (CFU-GM), but not HL-60/Ara-C cells from the growth inhibitory effects of agents such as thymidine, deoxyadenosine, deoxyguanosine, hydroxyurea and IMPY. Biochemical and cloning studies will be extended to include primary cultures of human leukemic myeloblasts obtained from patients refractory to high dose Ara-C regimens. An attempt will be made to correlate decreased deoxycytidine kinase activity in these cells with a specific vulnerability to the growth inhibitory effects of regimens employing pyrimidine antagonists in conjunction with deoxycytidine. Based upon these results, a Phase I study of high dose thymidine and deoxycytidine will be initiated in leukemia patients who have failed to respond to high dose Ara-C containing regimens. Pharmacokinetic studies will be utilized in order to achieve plasma thymidine and deoxycytidine concentrations capable of exerting a selectively lethal effect on Ara-C-resistant leukemic cells in the in vitro setting.